Normally a sewer service line is extended above ground several feet so that it can be located when a tie-in to the intended structure is desired.
The portion of the pipe above the ground, however, can be, and often is, struck by mowing or construction equipment. Such an impact can damage or break off the pipe, depending on the force of the impact. In such cases, water (e.g., from rainfall or other sources) may enter the pipe and make its way into the sewer collection system.